A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2020 Jan 17, 08:49 -0800
Laurence, you wrote:
"I rarely come up with an LOP under 20NM, and I can't figure out why."
Do you plot your LOPs? They're not necessarily bad just because the intercepts are long. If you're using Pub.229 or 249 (or equivalents) for sight reduction, they require a specially chosen assumed position which will typically be a dozen or two dozen miles from your actual position. The intercept distances, which are the values of Ho-Hc that you get at the end of the sight reduction, may often be as much as 20 nautical miles. This does not mean your sights are flawed in any way. They could be absolutely perfect, and you'll get results like this. When you plot the LOPs, you'll find they go right through your actual position.
Another option, as others have mentioned, is that there's an issue with the way you're subtracting the Sun's SD since you're doing artificial horizon sights. For an alternative to AH sights and a sanity check, do you have a small lake anywhere nearby? If you're able to sit on the ground right at the water's edge (height of eye maybe four feet?), then you'll have a proper "sea" horizon even on a lake as small as two or three nautical miles across. Even if there are hills or trees or buildings visible across the lake, you have a proper horizon for sextant sights if the distance across is greater than about 1.15 times the square root of the height of eye.
And you asked:
"Would someone look over a recent sight reduction of mine and see if I am doing something wrong?"
Of course! You'll get plenty of help here. Please consider posting two things: the raw sight data for five to ten recent sights, and also one or two recent sight reductions so that we can see what process you're using.
For raw sights, please include:
- the actual location where the sights were taken (if you're not sure of your exact lat/lon, go into Google Maps, e.g., find your observing location, right-click on the exact spot and select "What's Here?"),
- the name and any other details on the observed object. For example "Sun, center",
- sextant index error, if any,
- either height of eye or a notation that it's an artificial horizon sight,
- the date and time of each observation to the nearest second or two,
- the observed altitude of the body,
- any unusual weather conditions (no real concern if the body is above 15° altitude)